R.P.M Lathe meter mod.

[FEM2008]

Your first interpretation of that post was correct. The bicycle speedos allow the input of the bicycle wheel diameter, so by adjusting the input diameter you can alter the displayed reading for a given RPM of the thing being measured (within the range of acceptable input values). As the m/s value (or FPM) is directly proportional to the RPM of the bandsaw wheel it is only necessary to fudge the input wheel diameter for the readout to be in appropriate linear units. That is a trivial calculation. If the calculated fudged diameter is below what the bicycle speedo accepts then you can fudge the reading by integer multiples by the simple expedient of getting multiple pulses from each rev of the band saw wheel.

The above only applies to the use of the bicycle speedos (mentioned in post 3) or other tachos which allow for a method of calibration. IT DOES NOT APPLY to the type of tacho shown in the first post. These have no calibration or adjustment features, they are made to display RPM based on a single pulse per rev.

However, it would be a simple project to put an Arduino or similar micro between the pickup and display, programmed to convert the pulse rate into a linear measure which could be made m/s or FPM at the press of a button or flick of a switch. The programme (confusingly called a sketch in Arduino speak) could have the bandsaw wheel diameter hard coded before being downloaded from a PC. That would keep the programming simple but would mean that minor software changes would be needed for each wheel diameter or there are various ways to set it up to accept the diameter data via controls on the device itself, like programming a VFD or setting the time on a digital clock. The Arduino programming would be more complex in that case.

Adding a couple of buttons or even pot to the Arduino that can be used to vary a constant is not that hard. My new favorite way is use these super cheap encoders to set values https://howtomechatronics.com/tutori...s-use-arduino/ . But then, why use the Arduino as an intermediate device. You can simply implement the Arduino as the whole tachometer. All you need is the magnetic pickup.
Bicycle trip computer pickups typically use reed switches. These don't do well at high speed and might not work properly for a lathe or mill. A Hall sensor works much better A reflective photodetector is also excellent (example https://www.amazon.com/s?k=069525909...f=nb_sb_noss_1)

An Amazon link to a complete tach similar to what Tony Foal posted above (uses magnetic Hall sensor): https://www.amazon.com/EEEKit-Digita.../dp/B07PT7QPXM). You'll need a 12v external supply though.

[tonyfoale]

An Amazon link to a complete tach similar to what Tony Foal posted above (uses magnetic Hall sensor): https://www.amazon.com/EEEKit-Digita.../dp/B07PT7QPXM). You'll need a 12v external supply though.

One comment on the use of that type. They come supplied with a small disc magnet, the pickup is polarity sensitive so it is important which side of the magnet faces the pickup. There is no mention of this in the "instructions". I found out the hard way.

[Hans Pearson]

I have been thinking along the lines of a lathe rev counter and have thought of an electronic bicycle speedometer as I have used one on a wind speed indicator. I have tried this experimentally and it could work but there is quite a bit of calculating and setting that will have to be done, like determining the best wheel circumference to use (small would be better and the idea might only really work on slower lathes - Myford ML 7) I will come back on any further experiments or add to other trials that crop up here. Thanks for getting this thread going Captainleeward.